progress report on the 0 lifetime experiment …mcnudust/publication/presentations/...primex...

Progress Report on the π0 LifetimeExperiment (PrimEx) at JLab

Dustin E. McNultyMIT/JLab

PrimEx [email protected]

November 1, 2006

PrimEx Collaboration Jefferson Lab Hall B

Progress Report on the π0 LifetimeExperiment (PrimEx) at JLab

Outline

Physics Motivation

π0 Photoproduction Cross Section

Experimental Overview

Preliminary Compton Results

π0 Analysis Status

Summary and Outlook

Dustin McNulty, November 1, 2006, Hall A Post Doctoral Interview Seminar 1


PrimEx Collaboration

Institutions



Physics Motivation

π0 decay rate is a fundamental prediction of confinement scale QCD.

Presence of closed loop triangle diagramresults in nonconserved axial vector current,

Chiral Anomaly

even in the limit of vanishing quark masses.

In the leading order (chiral limit), the anomaly leads to the decay amplitude:

Aπ0 γγ

αem

4πFπεµνρσkµk

νε

ρε

σ (1)

or the reduced amplitude,

Aγγ

αem

4πFπ

0 02513 GeV

1 (2)

where Fπ

92 42

0 25 MeV is the pion decay constant.



Physics Motivation

The π0 γγ decay width predicted by this amplitude is

Γπ0 γγ

m3π

Aγγ

2

64π

7 725

0 044 eV (3)

Current Particle Data Book value is 7 84

0 56 eV

The above result for the decay amplitude is exact in the chiral limit,however for non-vanishing quark masses there are corrections:

Due to isospin sym-breaking (mu md), π0, η and η mixing induced.

Further corrections induced by terms in the Chiral Lagrangian.

Next to Leading Order prediction for the decay width is 8 10 eV

1%

Calc. using Chiral Perturbation Theory and 1/Nc expansion.J.L.Goity et al, Phys. Rev. D66, 076014 (2002); B.Moussallam, Phys. Rev. D51, 4939 (1995)

This is 4% higher than current experimental value!

A precision measurement of the π0 decay width is needed.



Physics Goal

Use the Primakoff effect to measure Γπ0 γγ to within 1 5% uncertainty

Experiments

π0 →γγ

Dec

ay W

idth

(eV

)

CERN(Direct)

Cornell(Primakoff)

DESY(Primakoff)

Tomsk(Primakoff)

PrimExExperiment

Leading OrderChiral Anomaly

Next to LeadingOrder, ±1%

7

8

9

10

11

12

0 1 2 3 4 5 6 7



The Primakoff Effect

π0 photoproduction fromCoulomb field of nucleus.

Equivalent production (γγ

π0)and decay (π0 γγ) mechanismimplies Primakoff cross sectionproportional to π0 lifetime.

Primakoff π0 produced at veryforward angles.

dσP

dΩ Γ

π0 γγ

8αemZ2

m3β3E4

Q4

Fem

Q

2sin2θπ (4)



Full Cross Section Componentsdσπ0

dΩ

dσP

dΩ

dσC

dΩ

dσI

dΩ

2 dσP

dΩ

dσC

dΩcos

φ

(5)

Primakoff Nucl.Coherent Incoherent Interference

Primakoff:Proportional to Z2,peaked at θπ0

m2π0

2E2γ

Nuclear Coherent:

dσC

dΩ

C A2

FN

Q

2sin2θπ (6)

Nuclear Incoherent:

dσI

dΩ

ξA

1 G

Q

dσH

dΩ(7)

Interference:



Experiment Overview

Conducted at Jefferson Lab, Fall 2004

Used 5 75 GeV continuous e ! beamand Hall B γ-tagging facility

Tagged photons incident on5%X0 targets: 12C and 208Pb

New PrimEx/Hall B calorimeter(HyCal), upstream of CLAS,designed to detect π0 decay γ’s

Measured 3 physical processes (absolute cross sections): Primary - π0

production, Secondary - Compton and e

"

e ! pair production

Improvements over previous experiments: Precision tagged γ flux andincident γ energy info, enhanced π0 angular and mass resolution, andidentification and subtraction of background event contamination



Experiment Overview



Hall B Photon Tagger

e-beam Photon-beam

PrimEx energy Range0.95 0.85

Single dipole magnet combined with a hodoscope containing twoplanar arrays of plastic scintillators to detect energy-degraded electronsfrom a thin bremsstrahlung radiator.

Tagger has 0 1% energy resolution and is capable of 50 MHz rates.



Photon Flux Control

PrimEx goal: Total uncertainty in photon flux

#

1 0%.

Number of tagged photons on target (Nγ) calibrated periodically usinga Total Absorption Counter (TAC).

Any drifts in the tagging ratio, occuring between calibration points, aremonitored online with the e

"

e ! pair spectrometer.



Hybrid Calorimeter – “HyCal”.

Optimal performance/costdesign

1 2 m $ 1 2 m, 1728 channels

576 Lead-glass (outer layers)

1152 Lead-Tungstenatecrystal (inner layers)

Lead-glass PbWO4

Energy Res. (∆E/E) 3 5 % 1 2 %

Position Res. (∆x,y) % 5 mm % 1 5 mm

Angular Res. (∆θπ0 ) % 675 µrad % 300 µrad



HyCal



HyCal Calibration

Full x,y motion allowed each ch. to be scanned through tagged γ beam.

Performed at both the beginning and end of the experiment.



Compton Scattering:

PbWO4

:(x,y,E’)γ

θγscattered photon

incident photon scattered atomic electron

Lead-Glass

Lead-Glass

e:(x,y,E)

beamline

HyCal

MeasuredMeasured Eo

Measured

A well known process, useful in several ways:

Detector/beam alignment

HyCal gain monitoring

Overall check of PrimEx setup to measure absolute cross sections

& Dedicated ”Double-Arm” Compton Runs:

Performed on a weekly basis, BPS

0, Ibeam

% 5 10 nA

Both e and scattered photon detected in HyCal

Compton Cross Section Measured: 12C and 0 5%X04Be

& “Single-Arm” Compton Data:

Dominant Source of Events in π0 production dataruns

BPS

% 2 T, Ibeam

% 100 nA, only scattered photon detected



Beam Alignment Monitoring using Single-Arm Compton(Y

-

Y) m

ean

(X

- X

) mea

nC

ompt

Com

pt

.

Only scattered γ measured

X ' reported HyCal coord

XCompt

' calc. (x,y) fromHycal E and Compton kin.

If beam alignmentperfect: (XCompt-X) = 0

Technique tracksalignment at 0 1 mm level

Jump in X correlated withbeamline BPM



Very Preliminary Compton Cross Section

0.2

0.21

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.3

4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6

Ebeam (GeV)

tota

l cro

ss se

ctio

n (m

b) p

er e

lect

ron

Klein-Nishina

PrimEx data, C12 target

0.2

0.21

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.3

4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6

Ebeam (GeV)

tota

l cro

ss se

ctio

n (m

b) p

er e

lect

ron

Klein-Nishina

PrimEx data, Be target

.



π0 Analysis Status: γγ Invariant Mass

110 120 130 140 150 160 170 180 190

Eve

nts/

0.5M

eV

0

2

4

6

8

10

12

14

310× Invariant Massγ Two

(MeV)γγm

C target12

detector4PbWO

) = 2.52 MeVγγ(mσ

110 120 130 140 150 160 170 180 190

Eve

nts/

0.5M

eV2

4

6

8

10

12

14

16

18

310× Invariant Massγ Two

(MeV)γγm

C target12

detector4PbWO+ Tagger Energy Constrain

) = 1.31 MeVγγ(mσ

.



π0 Analysis: Event Timing

trigPhoton.tdiff (ns)-40 -20 0 20 40

trigPhoton.tdiff (ns)-40 -20 0 20 40

Yie

ld /

ns0 π

10

210

310

410

pk_int 292± 8.214e+04

mean 0.004± -3.447

sigma 0.0027± 0.9732

slope 2.0e-05± 1e-05

intercept 2.6± 192.5

time (ns)Σtdiff = TAGM[0] time - HyCal total



π0 Analysis: Event Elasticity

tagger/EclusterPairE0.8 0.9 1 1.1 1.2

tagger/EclusterPairE0.8 0.9 1 1.1 1.2

Yie

ld/0

.010

0 π

0

2

4

6

8

10

12

14

16

310×

/ ndf 2χ 60.57 / 31

const1+2 301± 6.522e+04

mean1 0.000± 1.002

sigma1 0.00018± 0.01536

const2 1787± 1.369e+04

mean2 0.0031± 0.9744

sigma2 0.00118± 0.02461

p3 40± 1.142e+05

p2 50± -3.079e+05

p1 67± 2.611e+05

p0 48± -6.658e+04

301.2± = 65224 ctsPeak

tagger/EclusterPairElasticity = E

, Radiator BbeamAll I

tdiff cut: [-7.4,0.6] nsC, crystal only12

:[0.9,1.1] = 21120=>24.5%ctsBkgd



π0 Experimental Yield: 12C

(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3

’sγ/2

.403

e+12

inci

dent

o

Yie

ld/0

.02

0 π

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1310× C, crystal only)12 Photoproduction Yield (0π

yield0πElastic s = 649590πTotal

]= 5498os:[0,0.30π

Preliminary



π0 Experimental Yield Comparison: 12C



o Y

ield

/0.0

20 π

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1310× C)12-Yield (0πCalculated

(degrees)θ Production Angle, 0π

------ Coulomb ------ Nuclear Coherent------ Interference ------ Incoherent



o Y

ield

/0.0

20 π

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1310× C)12-Yield Comparison (0π


---- Calculated Experiment•



π0 Lifetime Extraction

Convert Yield to Cross Section.

dσexp

dθπ0

Nyieldπ0

θπ0

Nγ

$ Nt

$ επ0

θπ0

$ ∆θπ0(8)

where Nγ

( # of γ’s on target (preliminary uncertainty

)

1%).

where Nt

( target atoms/cm2 (thickness mapped to % 03% uncertainty).

where επ0

( experimental acceptance (uncertainty still being evaluated)

Fit experimental cross section with parameterization:

dσexp

dθπ0

bpT 2p

* bcT 2c

* biT 2i

* 2cosφ bpbcTpTc (9)

where the parameter bp

Γγγ

Vary the four parameters (bp, bc, bi, and φ) and minimize χ2.



π0 Experimental Cross Section Comparison: 12C



ob/

rad)

/0.0

2µ (θ

/dσC

ross

Sec

tion,

d

0

5

10

15

20

25

30

35C)12Calculated Total Cross Section (


------ Coulomb ------ Nuclear Coherent------ Interference ------ Incoherent



ob/

rad)

/0.0

2µ (θ

/dσC

ross

Sec

tion,

d0

5

10

15

20

25

30

35C)12Total Cross Section Comparison (


---- Calculated Experiment•



Comparison of +Independent π0 Analyses: 12C

(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5

(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5

b/ra

d)µ (θ

/dσE

xper

imen

tal C

ross

Sec

tion,

d

5

10

15

20

25

30

35C, Geom. Acc. Corrected Only)12 Photoprod. Cross Section (0π

PreliminaryI. Larin

D. McNulty



Summary and Outlook

High Quality precision π0 photoproduction data on 12C and 208Pb targetsusing 4 9

)

Etaggedγ

)

5 5 GeV has been collected and analyzed by thePrimEx Collaboration.

State of the art performance by the Hall B tagger and PrimEx calorimeter —delivering precision photon flux statistics combined with stellar energy andcoordinate resolutions.

Three % independent π0 analysis groups; two groups have achieved niceagreement, third group coming with comparison soon.

Preliminary Compton cross section results in good agreement with theory;final radiative corrections still pending.

Preliminary π0 results should be expected early next year–including crosssections and lifetime.



Extra Slide: Inelastic Bkgd Correction

(degrees)θ Production Angle, 0π0 0.5 1 1.5 2 2.5 3 3.5

)o B

ackg

roun

d Y

ield

(per

0.1

00 π

0

0.2

0.4

0.6

0.8

1

1.2

1.4

310×C, crystal only)12 (0πθ Inelastic Bkgd vs. 0π

hr6Entries 0

/ ndf 2χ 15.51 / 23

p0 9.819± 265

p1 45.35± -317.9

p2 40.56± 2330

p3 13.67± -1188

p4 4.186± -104.5

p5 1.199± 129.1

p6 0.3064± -15.23

/ ndf 2χ 15.51 / 23

p0 9.819± 265

p1 45.35± -317.9

p2 40.56± 2330

p3 13.67± -1188

p4 4.186± -104.5

p5 1.199± 129.1

p6 0.3064± -15.23

/ ndf 2χ 15.51 / 23

p0 9.819± 265

p1 45.35± -317.9

p2 40.56± 2330

p3 13.67± -1188

p4 4.186± -104.5

p5 1.199± 129.1

p6 0.3064± -15.23

/ ndf 2χ 15.51 / 23

p0 9.819± 265

p1 45.35± -317.9

p2 40.56± 2330

p3 13.67± -1188

p4 4.186± -104.5

p5 1.199± 129.1

p6 0.3064± -15.23

C, crystal only)12 (0πθ Inelastic Bkgd vs. 0π



Extra Slide: Inelastic Bkgd Correction


)o Y

ield

(per

0.0

20 π

0

0.2

0.4

0.6

0.8

1

1.2

310× C, crystal only)12 Photoproduction Yield (0π

yield + bkgd0πElastic

bkgd0πInelastic s = 213580πTotal

]= 833os:[0,0.30π

Preliminary



Extra Slide: π0 Experimental Yield: 208Pb

Production Angle (degrees)0π0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

’sγ Y

ield

/0.0

4 de

g/1.

377e

+12

inci

dent

0 π

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

310× Pb, all channels)208 Photoproduction yield (0π

Fri Nov 18 13:51:23 2005

yield0πElastic Preliminary

.



Extra Slide: π0 Experimental Yield: 12C

Production Angle (degrees)0π0 0.5 1 1.5 2 2.5 3 3.5 4

’sγ Y

ield

/0.0

4 de

g/3.

439e

+12

inci

dent

0 π

0

0.5

1

1.5

2

2.5

3

310× C, all channels)12 Photoproduction yield (0π

Mon Oct 31 13:29:41 2005

yield0πElastic Preliminary

.


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